[inductor] Refactor device guard Python codegen to allow nested indentation (#119673)
Summary: The codegen of `with torch.cuda._DeviceGuard` context manager in the Python wrapper code is implemented via `device_cm_stack: contextlib.ExitStack()`. As the context managers in the stack are `code.indent()`, this means that the whole stack is unindented at once on `device_cm_stack.close()`. This becomes problematic when attempting to codegen indented code (e.g., for control flow in Python and / or nested subgraph codegen-ing).
In this PR, we refactor the device guard codegen-ing in Python by replacing the `device_cm_stack` by explicit indent and unindent calls for entering and exiting the `with torch.cuda._DeviceGuard` context manager. This allows for nested device guard context managers and better aligns with other indented codegen-ing intertwined with it (e.g., for nested subgraph codegen-ing).
This is necessary for the upcoming support for `torch.cond` (and other control flow operators) in Inductor. Before that, the only change in the Python wrapper codegen is that the `return outputs` is now happening outside the `with torch.cuda._DeviceGuard` context manager.
Test Plan: CI
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119673
Approved by: https://github.com/peterbell10
